1. Field of the Invention
The present invention relates to an optical information processing apparatus for recording and/or reproducing information on/from an optical recording medium such as an optical disk.
2. Related Background Art
Recently, a "digital signal processing method" of performing digital signal processing in place of conventional analog processing has become popular, and is widely used in practical applications such as compact disks (CDs), digital audio tapes (DATs), communication lines, and the like. The "digital signal processing method" has many merits since a complicated signal processing algorithm can be realized by software. For example, a hardware system can be simplified, system cost can be reduced, changes in specifications such as selection of filter constants, algorithms, and the like can be flexibly made, and so on. In addition, along with the progress in IC techniques, inexpensive digital signal processing integrated circuits (ICs), and digital signal processors (DSPs) are commercially available.
An optical information processing apparatus such as an optical disk has rapidly progressed as a high-density, large-capacity memory from a compact disk (CD) player to a rewritable magnetooptical disk apparatus via a direct-read-after-write (DRAW) type apparatus. In particular, a magnetooptical disk apparatus is required to have high reliability and a high access speed in addition to the above-mentioned high-density, large-capacity features as an external memory of a computer.
For this reason, an electrical servo system requires control as a complicated combination of outputs from various sensors. A magnetooptical disk apparatus will be exemplified below with reference to FIG. 1.
In FIG. 1, a light beam emitted from a semiconductor laser 1 is collimated by a collimator lens 2, and the collimated beam is converted by a polarization beam splitter 3 with a beam shaping function into a light beam having a substantially circular section. The collimated beam is reflected by a prism 4, and is then incident on an object lens 5. The object lens 5 is movable in a focus direction 6 and a tracking direction 7 by an actuator (not shown), and focuses a small light spot on a disk 9. A magnetooptical recording layer is formed on the disk 9. A direction of an arrow 10 corresponds to a direction of tracks, and an arrow 11 indicates the center of rotation of the disk. The prism 4, the object lens 5, the actuator, and the like are fixed on a carriage 13, and the carriage 13 can be moved in a radial direction of the disk 9 using a linear motor (not shown), and the like.
Light reflected by the disk 9 is converted to a collimated beam by the object lens 5, and is deflected by the prism 4 toward the polarization beam splitter 3. The light beam is then reflected by the polarization beam splitter 3 toward a direction of a detection optical system, and is split by a beam splitter 16 through a focusing lens 15 into a light beam reflected toward a servo sensor 18 and a light beam transmitting toward radio frequency (RF) sensors 19 and 20.
The focusing lens 15 includes an element for generating, e.g., an astigmatism, and the light beam is focused on the servo sensor 18. The servo sensor 18 comprises four-split sensors 18-1 to 18-4. The servo sensor 18 is aligned in directions of three axes while observing that a light spot is focused on a predetermined track on the disk 9. The sensor 18 is then adjusted to obtain equal outputs from the four sensors.
The light beam transmitting through the beam splitter 16 is split into two beams by a polarization beam splitter 17, and the two beams are respectively focused on the RF sensors 19 and 20. The semiconductor laser 1, the collimator lens 2, the RF sensors, and the like are fixed to a head fixing unit 14. The magnetooptical disk apparatus shown in FIG. 1 employs a so-called divided optical system which is divided into the carriage 13 and the head fixing unit 14, and can allow high-speed access.
An actuator unit of the magnetooptical disk apparatus will be described below with reference to FIG. 2.
In FIG. 2, the object lens 5 is fixed to a bobbin 21. A tracking coil 22 and a focusing coil 23 drive the bobbin 21 in the tracking and focus directions 7 and 6 in cooperation with a tracking magnet 24 and a focusing magnet 25 fixed to a yoke 26. The bobbin 21 is supported by a support shaft 27. An under limiter 28 determines the lowermost end of the bobbin. A counterweight 29 of the object lens 5 is fixed to the bobbin.
A light-emitting diode 30 is fixed to a flexible printed board 31. A light beam emitted from the light-emitting diode 30 is shaped via a slit 32, and the shaped light beam is projected onto a 2-split sensor 34 as a light beam 33. The light-emitting diode 30 is fixed to the bobbin 21. When the actuator is shifted in the tracking direction, amounts of the light beam 33 incident on light-receiving surfaces 34-1 and 34-2 of the 2-split sensor are changed, and the outputs from these surfaces can be calculated to detect a position of the object lens 5. The 2-split sensor 34 is connected to a flexible printed board 35.
A linear motor unit of the magnetooptical disk apparatus will be described below with reference to FIG. 3.
In FIG. 3, an actuator including the object lens 5, the bobbin 21, the magnet 24, the yoke 26, and the bobbin support shaft 27 is fixed on the carriage 13. The carriage 13 is supported on rails 36-1 and 36-2 through, e.g., bearings 37-1 and 37-2, and is movable in a disk radial direction 12. The linear motor unit comprises a coil 38, a yoke 39, magnets 40-1 and 40-2, and the like. In this case, linear motors are attached to two sides of the carriage to allow high-speed access. A spindle motor 41 rotates the disk.
A servo system for the magnetooptical disk apparatus described above with reference to FIGS. 1 to 3 will be described below with reference to FIG. 4.
The servo sensor 18 is adjusted to obtain equal outputs from the four sensors 18-1 to 18-4 when the object lens 5 is located at the center of a light beam from the semiconductor laser 1 and the light beam forms a small spot of about 1 micron on a track of the disk 9. In this case, since a focus error detection method employs an astigmatism method, if the outputs from the sensors 18-1 to 18-4 are represented by S.sub.1 to S.sub.4, a difference between outputs of diagonal sums is observed, thus obtaining a focus error signal S.sub.AF given by: EQU S.sub.AF =(S.sub.1 +S.sub.3)-(S.sub.2 +S.sub.4)
For example, when a light spot is in an in-focus state on the disk, the above-mentioned output becomes 0. When the light spot is in a near-focus state on the disk, a negative output is obtained; when the light spot is in a far-focus state, a positive output is obtained.
A tracking error detection method employs a push-pull method. In the push-pull method, a balance of diffraction light from a guide groove of a disk is observed in a far field. A distribution of diffraction light is unbalanced according to a radial position shift between a predetermined track on a disk and a light spot. Thus, a difference between outputs of the sensors divided by a dividing line along a tangential direction of the sensor 18 is observed, and a tracking error signal S.sub.AT given by the following equation is obtained: EQU S.sub.AT =(S.sub.2 +S.sub.3)-(S.sub.1 +S.sub.4)
For example, when the light spot is located on a track, the output is zero. When the light spot is shifted in an inner peripheral direction of the disk, a negative output is obtained; when the light spot is shifted in an outer peripheral direction of the disk, a positive output is obtained.
In the push-pull method, when the object lens 5 is largely shifted in a radial direction (tracking direction) by, e.g., a multi-track jump mode, since the light beam focused on the servo sensor 18 is moved in the radial direction, an auto-tracking (to be abbreviated as AT hereinafter) output is offset in addition to an unbalanced distribution of diffraction light according to the above-mentioned track shift. In order to perform high-speed access, it is advantageous that the object lens can be used to be moved by about 100 to 150 tracks from the center of a light beam from the semiconductor laser 1. Since this offset almost corresponds to a shift amount of the object lens from the center of a light beam, it can be easily corrected as long as the object lens position can be detected.
In this case, the object lens position detection means (to be referred to as a lens sensor hereinafter) as described in FIG. 2 is arranged. The outputs from the two sensors 34-1 and 34-2 are respectively represented by S.sub.LP1 and S.sub.LP2, and these outputs are adjusted so that a lens position (to be abbreviated as LP hereinafter) output S.sub.LP given by the following equation becomes 0 when the object lens is located at the center of the light beam: EQU S.sub.LP =S.sub.LP1 -S.sub.LP2
When the object lens 5 is located at the center of the light beam, the above-mentioned output becomes 0. However, when the object lens is shifted in an inner peripheral direction of the disk, a positive output is obtained; when it is shifted in an outer peripheral direction of the disk, a negative output is obtained.
Since the LP sensor output represents a position shift between the carriage 13 and the object lens 5, a linear motor can be driven using this data, so that the object lens position can always be kept at the center of the light beam.
The servo sensor and the like have been described. However, it is impossible to perfectly mechanically align these components. When conventional analog servo signal processing is performed, a sensor output is normally electrically adjusted by a control volume (not shown) after mechanical adjustment.
Servo signal processing will be briefly described below.
The outputs S.sub.1 to S.sub.4 from the servo sensor 18 are amplified by a preamplifier 43, and then are output from an arithmetic unit 44 as AT and auto-focus (to be abbreviated as AF hereinafter) outputs, as described above. The outputs S.sub.LP1 and S.sub.LP2 from the LP sensor 34 are amplified by a preamplifier 45, and then are output from an arithmetic unit 46 as the LP output. Of these outputs, the AT and LP outputs are added to each other by an adder 47 to be corrected, so that the tracking error signal is not offset even if the object lens position is shifted (corrected AT output). The AF output, the corrected AT output, and the LP output are supplied to a digital signal processing circuit 48, and are then output respectively to AF, AT, and linear motor drivers 49, 50, and 51 at proper timings. These drivers output drive signals to the AF, AT, and linear motor coils 23, 22, and 38, respectively, thus executing focus control and tracking control.
An RF system will be described below with reference to FIG. 5.
The RF system shown in FIG. 5 includes the RF sensors 19 and 20 described above. Preamplifiers 52 and 53 respectively amplify outputs from the RF sensors 19 and 20. Amplifiers 54 and 55 calculate a difference and a sum of the outputs from the RF sensors 19 and 20. A magnetooptical signal output 56 is detected as a difference between the outputs from the RF sensors 19 and 20 in such a manner that rotation of a plane of polarization of a light beam caused by a magnetooptical effect is detected by the polarization beam splitter 17. A preformat signal 57 representing, e.g., a sector mark or an address corresponds to a linear increase/decrease in light amount incident on the RF sensors 19 and 20, and is detected as a sum of the outputs from the RF sensors 19 and 20.
A magnetooptical disk will be described below with reference to FIG. 6.
In FIG. 6, tracks 58 and guide grooves 59 are concentrically or spirally arranged to have a disk center 11. Each track is divided into header areas in each of which preformat signals such as sector marks and addresses are recorded in advance in the form of pits 60, and data areas in each of which magnetooptical signals are recorded by a user in the form of magnetooptical pits 61.
In the magnetooptical disk apparatus with the above-mentioned arrangement, since sensor outputs are electrically adjusted by, e.g., an adjusting volume, this results in a cumbersome operation, and it is difficult to reduce manufacturing costs of the apparatus.
Since the above focus or tracking error detection method does not directly detect a focusing state of a light spot on a disk, when a relative position between the servo sensor and a light spot on the servo sensor or a position of the semiconductor laser is shifted by any external force after adjustment, the light spot can no longer be correctly focused on a predetermined track. These changes in states may often occur due to a change in temperature or a vibration during transportation, and servo precision is impaired. Even when the position sensor itself does not suffer from a position shift, the wavelength of the semiconductor laser may be changed due to a change in temperature. As a result, a deflection angle of a light beam in the polarization beam splitter with the beam shaping function is changed, and a light spot position on the servo sensor is undesirably moved.
An AT offset which occurs when the center of the object lens is shifted from an optical axis in a so-called multi-track jump mode wherein the object lens is moved in the tracking direction to move a light beam to another track separated from the present track by several tracks is changed due to a variation in depth of the guide groove. Therefore, this results in degradation of tracking precision unless an offset value is adjusted for every disk.
Japanese Laid-Open Patent Application No. 53-129604 discloses an optical information processing apparatus which can automatically correct an AF offset.